Now obviously this was a death by a thousand cuts situation and there's no shortage of reasons you can point to why the game failed, so much stupid shit like microtransactions and battle passes that ruined what could have been a comfy Smash alternative. But the thing that bugs me the most about all the Smash-likes is how they see all the people playing with items off, not caring about the story mode, etc. And they say "ok, we just won't make any of that at all." And you end up with an empty game that has nothing to do. Every single one of them has been absolutely barren compared to Smash.
What I like about Smash is that Sakurai doesn't seem to give a fuck that people don't care about items, he still puts them in every single game anyway. So every game ends up packed with a bunch of content because he ignores the competitive players and does whatever he wants.